Freehand localization and biopsy of small, deep-seated, intracranial targets, often in discreet and sensitive areas, can be very difficult and unsafe. However, it has been possible to define and correlate the position of single or multiple points within the cranium with preoperative imaging and by the use of stereotactic frames known in the art. One known technique helps define the position of an intracranial target by coordinates derived from a frame that is rigidly fixed to the skull during preoperative MRI or CT imaging and during the surgical procedure.
More recent technological advances have led to the development of image guidance systems that do not require frame-based coordinates for target localization. One such "frameless" stereotactic guidance system is the MAYFIELD/ACCISS Stereotactic Workstation which is commercially sold the assignee of this application.
Frameless systems provide constant intraoperative navigational information, which permits a surgeon to identify precisely the spacial position of a hand-held probe in the surgical field with reformatted CT or MRI shown on a high definition display monitor. Unlike stereotactic head frames, therefore, these frameless systems allow the surgeon to locate multiple targets in different spacial planes.
The localization of an intracranial target with a stereotactic frame is achieved by advancing a probe along a predetermined linear trajectory that is usually fixed directly to the patient's skull. Systems do not carry structure fixed to the patient's head. Therefore, in using a hand-held localizing probe, much more of the steadiness dependent on the surgeon's hand. For this reason, fixed trajectory target sockets have been devised. These devices allow a surgical instrument to be introduced to intracranial target along a stable and fixed trajectory. The trajectory remains fixed relative to the intracranial target during the procedure, thus eliminating the risk of misdirection or drift associated with freehand procedures.
One such device is shown in assignee's U.S. Pat. No. 5,810,712, which is expressly incorporated by reference herein. One of this device includes of a target ball with a centrally axially drilled hole, the target ball being locked in a fixed position with, for example, a pair of hinged plates defining holes in which the target ball is sandwiched. Once the target ball hole is aligned with the intracranial target, the plates are tightened upon the target ball so that a surgical instrument may be inserted through the target ball hole and proceed along its fixed trajectory to the intracranial target.
Another such device for fixing the trajectory of the surgical instrument for intracranial procedures is the MAYFIELD/ACCISS AccuPoint targeting sphere available from the assignee of the present invention. In this system, a generally circular frame receives a targeting sphere therein, the targeting sphere then being locked upon a desired trajectory by a plurality of screws. These screws are introduced into the frame via threaded screw bores, wherein each screw must be independently tightened upon the targeting sphere before the targeting sphere is securely fixed in space so that a surgical instrument may be introduced along the desired trajectory. The MAYFIELD/ACCISS system is supported in space via a connecting arm to a Budde.RTM.-Halo retractor, which defines a work station above a patient's head.
However, while these prior art devices have proved suitable for many types of surgery, there is room for improvement. For example, when using multiple screws, each of which must be tightened separately upon the target ball, during tightening, upon the target ball there is a possibility the desired trajectory may become offset slightly from the desired trajectory. It is therefore desirable to be able to secure a target ball in space in such a manner so that a desired trajectory to an intracranial target is maintained during the act of tightening the target ball in place, with a reduced potential for "play" during tightening.
Also, in prior art devices, it is difficult and time consuming to loosen the target ball from its fixed position, re-position the target ball to a new desired trajectory, and then tighten the target ball on its new desired trajectory. Therefore, it is also desirable to be able to easily tighten and loosen the target ball in order to change trajectories more quickly and easily in the relatively cramped spaces of a surgical operating room.